A TRANSMISSION AND ANALYTICAL ELECTRON-MICROSCOPE STUDY OF EXSOLUTIONMICROSTRUCTURES AND MECHANISMS IN THE ORTHOAMPHIBOLES ANTHOPHYLLITE AND GEDRITE

An investigation of exsolution microstructures in 17 orthoamphibole sa mples (anthophyllite and gedrite) has been carried out using transmiss ion and analytical electron microscopy (TEM and AEM). All the amphibol es studied, even those appearing to be optically homogeneous, contain exsolution lamellae. TEM observations show a wide variety of exsolutio n microstructures, ranging from extremely coarse lamellae over 200 nm in thickness to very fine scale, homogeneously distributed Guinier Pre ston (GP) zones. Many samples show evidence of progressive exsolution during slow cooling of the samples. Evidence for heterogeneous nucleat ion and growth of lamellae is abundant, with (100) stacking faults and (010) chain-width errors (CWEs) as common nucleation sites. Heterogen eous nucleation has also been observed at grain boundaries, dislocatio ns, and microfractures and along the interfaces of oxide inclusions. T EM images suggest that the incoherent terminations of (010) CWEs and ( 100) stacking faults are regions of significant structural distortion and lattice strain. These areas of high local strain energy appear to be responsible for unusual lamellar morphologies consisting of embayme nts in the lamellae caused by boundary pinning of the lamellar interfa ces. In addition, there is considerable bulging of lamellae in the vic inity of (010) CWE terminations, which may result from enhanced chemic al diffusion along the structural tunnels associated with the defect t erminations, as well as from strain effects. In many samples, heteroge neous nucleation and growth was followed by homogeneous nucleation of smaller platelets in the solute-depleted regions between the larger la mellae. The most common lamellar orientation was found to be (010). Ot her lamellar orientations have also been observed, including curved la mellae straddling (010), (140), (130), and (120). Calculations using t he three-dimensional lattice fitting program, EPLAG, for intergrown an thophyllite and gedrite indicate that the combination of DELTAb and DE LTAa between the amphiboles controls the actual lamellar orientations. If DELTAb dominates over DELTAa, then a (010) optimal phase boundary is predicted. As DELTAb decreases and DELTAa increases, the optimal ph ase boundary gradually shifts from (010) to (120). AEM of the observed (120) exsolution microstructures suggests that increased Ca content i s largely responsible for this orientation. The differences in lattice parameters are also sensitive to the Na content and Fe/Mg ratio. AEM analyses of exsolved pairs of anthophyllite and gedrite indicate that edenite and tschermakite substitutions are of central importance in co ntrolling exsolution. The exsolved pairs define the widest gap found t o date for coexisting orthoamphiboles. Plotting the AEM data on the T- X solvus diagrams of Spear (1980) suggests that exsolution typically t ook place between 460 and 520-degrees-C.